High power semiconductor lasers, because of their small physical dimensions and high electrical power input, require special thermal management considerations. In addition to effectively extracting the waste heat (net of electrical power input and optical power output) from the active semiconductor laser region, considerable attention needs to be paid to ex-chip thermal management. The waste heat needs to be removed from the semiconductor laser mounting surface efficiently so that the active region of the laser remains at the proper operating temperature. Typical semiconductor laser package assembly involves bonding of the laser to a submount (substrate material) that, in turn, is mounted on a heat spreader or a heat sink.
Three critical parameters need to be considered when choosing the substrate (submount) on to which the semiconductor laser chip is mounted. The first is the thermal conductivity of the submount, which needs to be as high as possible to remove the heat efficiently from the laser. The second is the coefficient of thermal expansion (CTE) of the submount with respect to the coefficient of thermal expansion of the semiconductor laser material. The CTE of the submount must be close to that of the laser to minimize mechanical stress in the laser chip during large temperature cycles, both during the operation of the laser but also during the mounting process which, for reliability purposes, typically uses AuSn eutectic or other hard solder. Third, it is inconvenient to mount the chips directly onto heat spreaders and heat sinks because the heat spreaders are usually bulky. Furthermore, it is much easier to do proper metallization for laser interconnections on a compact submount wafer that is subsequently diced into individual submounts.
One of the most widely employed and technologically important submount materials for InP-based semiconductor lasers is aluminum nitride (AlN). Its CTE, being 4.5×10−6 K−1, is perfectly matched to that of indium phosphide (InP) at room temperature, essentially eliminating thermally induced mechanical stresses on the laser structure, both those frozen-in after die mounting and those arising from thermal cycling during device operation. In addition, high-grade AlN exhibits high thermal conductivity of ˜200 W/mK at room temperature. This value is three times higher than thermal conductivity of InP (70 W/mK), which naturally enhances heat transfer from the laser active region into the heatsink. Finally, AlN is an electrical insulator, and hence it permits easy formation of electrical interconnects on the submount for incorporating various passive monitoring elements into the integrated chip on carrier assembly. Due to the advantages listed above, AlN is one of the most widely used materials for semiconductor laser submounts. Similar combinations of submount materials and semiconductor laser materials can be found for other situations.
However, even though AlN has a relatively high thermal conductivity, higher laser performance can be achieved employing submount materials with even higher thermal conductivity. This is especially true for high-power continuous-wave quantum cascade lasers (QCLs) that generate up 30 W of waste heat, from a surface area typically 200 μm wide and 5-7 mm long, and whose performance deteriorates rapidly as the active region temperature rises.
Diamond has the highest known thermal conductivity (>1800 W/mK) of all natural materials, exceeding that for AlN by more than a factor of nine. Therefore, employment of diamond submounts is expected to significantly reduce laser active region temperature under the same laser driving conditions. However, the CTE of diamond at room temperature is ˜1×10−6 K−1. Thus, the CTE mismatch between InP and diamond is very large. So, to avoid the undesirable mechanical stress, it is necessary to use soft solders, such as indium, for laser bonding. Soft solders suffer from electromigration from the bonding area to the laser facets, which eventually leads to electrical shorts, facet damage, and eventual laser destruction. This significantly reduces laser reliability. Therefore, diamond is rarely used as a submount material for commercial semiconductor laser applications.
We identified another material that is significantly superior to AlN for high heat waste applications, namely silicon carbide (SiC). SiC has a thermal conductivity of up to 600 W/mK, three times that for AlN. As with AlN it can be made semi-insulating, which facilitates all the electrical interconnects. In addition, its CTE mismatch with InP can be less than 1.0×10−6 by proper choice of orientation and structure of SiC. It is worth noting that this mismatch is almost a factor of five smaller than that of InP on diamond. But even this relatively small CTE mismatch can still lead to reduced laser reliability, especially when the final assembly undergoes large thermal cycles during packaging and subsequent laser operation.